A method for measuring a rotation axis based on a spliced vortex beam

Abstract

The present application relates to a kind of based on splicing vortex light beam's rotation axis measurement method.Based on the rotation Doppler effect of complete vortex light beam has been widely used in rotational speed measurement, but all cannot determine the rotation center of the object to be detected, non-complete vortex light beam breaks the circular symmetry of light beam, so it can be sensitive to the rotation axis position of the object to be detected.On this basis, by loading non-complete vortex light beam with different topological charge in different azimuth, a kind of based on splicing vortex light beam's rotation axis measurement method is proposed.First, the polarization direction is adjusted using polarizer, linearly polarized light beam is generated, then the phase structure of hologram is adjusted, and splicing vortex light beam hologram with different topological charge in different azimuth range is obtained, loaded on spatial light modulator, then linearly polarized light beam is irradiated on spatial light modulator to generate splicing vortex light beam, used for detecting rotating object, the azimuth of object rotation axis is determined by collecting echo signal and using the different frequency spectrum distribution obtained by computer processing.The method can flexibly control the topological charge in different azimuth, realize simple device, high detection efficiency, and has great application value for determining the position of object rotation axis in actual measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of based on splicing vortex light beam rotation measurement method.The present application relates to holographic calculation, vortex light preparation, optical detection field, using the method of complex amplitude modulation generates new splicing vortex light beam for rotating object detection, by analyzing the echo spectrum obtained by once illumination, the direction of object rotation axis can be determined. BACKGROUND

[0002] Vortex light beam is a kind of special structure light beam with helical wave front and phase factor Since 1992, vortex light beam has the characteristic of orbital angular momentum, which has been discovered by L.Allen et al., and has received extensive attention from scholars at home and abroad. Currently, vortex light beams have been applied in many fields such as rotation detection, ultra-high capacity optical communication, astronomical measurement technology, etc. Among them, in remote sensing detection, vortex light beams show great potential. Based on the rotation Doppler effect of vortex light beams, the traditional Doppler effect can only detect the translational motion velocity parallel to the light beam, and has no effect on the rotational motion of the object surface. However, the rotation Doppler effect can effectively detect the angular motion of the object, produce frequency shift, and thus measure the rotation speed of the rotating object, which greatly makes up for the defects of traditional radar detection.

[0003] However, most of the current research on rotation Doppler effect uses complete vortex light beams, which can only detect the rotation speed, direction and other information of the object, and cannot locate the position of the rotation axis. Non-complete vortex light beams, while retaining their own orbital angular momentum characteristics, break the circular symmetry of the light beam, can be sensitive to the direction of the object, and can obtain new motion information of the object in the object rotation speed detection, which helps to solve the problem of determining the rotation center of the object. Although some previous research on vortex light beams for determining the center of the object uses non-complete vortex light beams, the determination process needs to change the parameters of the light beam and measure multiple times, which is complex and time-consuming. The motion state of the object may have changed during the measurement process, resulting in low measurement accuracy and poor positioning effect. To solve this problem, the present application uses three non-symmetrical defect superposition state vortex light beams with different initial azimuth angles and different topological charges to construct a splicing vortex light beam as a light source to illuminate the rotating object. Through single detection, the direction of the object rotation axis can be quickly obtained. Compared with the previous rotation axis determination scheme, this method is simple to operate, has great practical value, and greatly improves the efficiency of determining the position of the rotation center of the object in remote sensing detection. SUMMARY

[0004] The technical solution of the present application is: aiming at the problem of determining the center of the rotating object rotating shaft, a method for realizing rotating shaft azimuth measurement by single detection of new spliced vortex light beam is proposed.

[0005] The technical solution of the present application is:

[0006] The present application relates to a rotating shaft measurement method based on spliced vortex light beam:

[0007] (1) Firstly, the phase structure of light beams in different azimuth angle ranges is changed to obtain a spliced vortex light beam hologram. A spatial light modulator loaded with optical vortex hologram is irradiated by a polarized light beam to obtain a spliced vortex light beam. The spliced vortex light beam is used to irradiate a rotating object. A photoelectric detector is used to receive scattered light signals. After being collected by a collection card, the signals are transmitted to a computer for filtering, Fourier transform and other processing to obtain a rotating Doppler spectrum. The position of the rotating shaft of the object is determined according to the expression of the spectrum value and the angle of the rotating shaft center.

[0008] (2) In the aspect of spliced vortex light preparation, the phase structure of the hologram needs to be changed in different azimuth angle ranges, and the topological charge number in the vortex hologram is adjusted to obtain a spliced vortex light beam hologram. After the laser output light is expanded and collimated by a 4f system and polarized by a polarizer, it is irradiated onto a spatial light modulator loaded with a new spliced vortex light beam hologram. The first-order diffraction light with good light intensity and mode purity is selected as the spliced vortex light beam required for detection.

[0009] (3) In the aspect of object rotating shaft azimuth identification, firstly, based on the non-coaxial rotating Doppler effect, an expression between the rotating Doppler spectrum value of the spliced vortex light beam and the azimuth angle of the object rotating shaft is established. Then, the spliced vortex light beam is used to detect the rotating object. The scattered light is collected by a photoelectric detector. After the signal processing, the rotating Doppler spectrum is obtained. Combined with the change relationship between the spectrum and the azimuth of the rotating shaft, the azimuth of the object rotating shaft is determined.

[0010] The principle of the present application is:

[0011] The electric field expression of a complete vortex light beam in cylindrical coordinates is

[0012]

[0013] In the formula, is the light beam amplitude, where is the azimuthal angle, r is the radial distance, z is the propagation distance, k = λ / 2π is the wave number, and l is the topological charge. Based on equation (1), the phase and amplitude information of the vortex beams in different azimuthal ranges in the hologram are adjusted by using the complex amplitude modulation method, and three non-complete vortex beams with different topological charges in different azimuthal ranges are obtained, which are spliced into a spliced vortex beam, and the expression is:

[0014]

[0015] wherein is the unobstructed fan-shaped azimuthal value of a certain topological charge beam, is three different azimuthal angles, is a step function, is greater than or equal to The final spliced vortex beam uses its own asymmetric defects and multi-topological charge characteristics to realize sensitive identification of the rotating object's rotation axis.

[0016] When the vortex beam collimates and irradiates a rotating object with a rotation speed of Ω, a rotating Doppler shift will occur. According to the conservation of energy and angular momentum before and after the interaction between light and matter, a rotating particle with a rotation speed of Ω0 will generate a rotating torque of to each light particle with orbital angular momentum, which will transfer an energy of to each photon, where l is the topological charge, is the Planck constant, so that the scattered light signal produces a single frequency shift value Δf, which can be expressed as

[0017]

[0018] When the beam propagation axis and the object rotation axis have a lateral shift d, in order to quantitatively analyze the frequency shift value of the echo signal, a Cartesian coordinate system is established with the center of the vortex beam as the origin and the positive half of the x-axis as the starting position of the azimuth angle. The angle of each micro scattering point is represented by , and the position of the object rotation axis relative to the center of the beam is determined by θ, as shown in Figure 2 According to the geometric relationship in the figure, we can get:

[0019]

[0020] where d is the lateral shift distance between the object rotation center and the beam center, R is the distance between the scattering point on the spot and the rotation center, and r is the radius of the vortex beam. From equation (4), the angle γ between the linear velocity v of the object particle and the tangent direction of the vortex beam is

[0021]

[0022] Decompose the linear velocity v along the tangent direction of the cross section of the vortex beam and the radial direction of the line connecting the center of the beam and the particle, to obtain the expression of the angular velocity Omega0 of the rotating particle transferred to the light particle,

[0023]

[0024] The final rotation Doppler shift Delta f generated by the scattered light particles at different azimuth angles is,

[0025]

[0026] Where l is the topological charge number, and the frequency spectrum obtained by finally splicing the vortex beam is determined by the different coverage ranges of the incomplete vortex beam And the central angle theta of the object rotation, when the object is at different positions, the value of theta changes, and the incomplete vortex beam with different topological charge numbers only exists in a limited and fixed angle range Therefore, the change of the peak value and the spread of the rotation Doppler shift generated by the spliced vortex beam is only related to the position of the rotation axis. When the above spliced vortex beam is used as the incident beam, the topological charge number at different azimuth angles is different, and the central frequency shift value generated is also different. The rotation Doppler shift appears three peaks, and the peak values at the three different frequency shifts will change differently when the azimuth angle theta of the rotation axis is different. According to the relationship between the angle theta and the change of the three peaks, the azimuth of the rotation axis of the object can be determined.

[0027] Compared with the prior art, the main advantages of the present application are:

[0028] (1) The optical path of the present application is simple and easy to operate. By loading a spliced vortex beam hologram that has been modulated on a spatial light modulator, the generation of the beam and the detection of the rotation axis of the object can be realized. The hologram does not need to be frequently replaced and the optical path needs to be adjusted.

[0029] (2) The detection efficiency of the present application is high. Only one detection is needed to determine the azimuth of the rotation axis of the object. The determination of the rotation center of the object needs to use the vortex beam for multiple detections. Compared with the previous method, the present application can determine the position of the rotation axis only by one detection data, which greatly improves the detection efficiency.

[0030] (3) The present application has good robustness, and the topological charge number of the spliced beam and the azimuth of the incomplete vortex beam can be flexibly changed to determine the rotation axis center of the rotating object under different conditions. For example, when the propagation axis of the beam is greatly offset laterally from the rotation axis of the object, the topological charge number interval of the spliced vortex beam can be appropriately increased to make the frequency spectrum more clear and more accurately determine the position of the rotation axis. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1Flow chart for rotation axis measurement based on spliced vortex beam

[0032] Figure 2 Schematic diagram of rotational Doppler effect based on scattering point model

[0033] Figure 3 Schematic diagram of experimental light path for verification

[0034] Figure 4 Intensity diagram of spliced vortex beam with different topological charge interval

[0035] Figure 5 Intensity diagram of spliced vortex beam with different azimuthal angle interval

[0036] Figure 6 Intensity diagram of multi-topological charge spliced vortex beam

[0037] Figure 7 Signal diagram of rotational Doppler frequency shift when rotation axis is in the first quadrant

[0038] Figure 8 Signal diagram of rotational Doppler frequency shift when rotation axis is in the second quadrant DETAILED DESCRIPTION

[0039] The implementation object of the present application is a spliced vortex beam and a rotational Doppler frequency shift value. The specific implementation steps are as follows: first, according to formula (2), set the azimuth angle respectively 0, π, 3π / 2; The topological charge l1, l2, l3 in the range of [0, π / 2], [π, 3π / 2], [3π / 2, 2π] is ±20, ±60, ±40 respectively, the light field hologram is designed according to the complex amplitude modulation method, and a spliced vortex beam hologram is constructed. The rotation axis azimuth detection light path diagram is as shown in Figure 3 The Gaussian beam emitted by the laser is modulated into linearly polarized light by the polarizer, and is irradiated onto the spatial light modulator loaded with the new spliced vortex beam hologram. In order to make the orbital angular momentum mode purity and intensity of the emitted vortex beam highest, the diffracted beam passes through the 4f system composed of lenses L3, AP and L4 in turn, selects the first order diffracted light, and obtains the spliced vortex beam, as shown in Figure 2, then through the half-transmission half-reflection mirror (BS) is irradiated on the rotating object, the reflected light signal is collected by the BS, the convex lens is collected by the avalanche photodetector (APD) and is converted into an electric signal, then after being collected by the collection card, the data is transmitted to the computer for processing to obtain the rotating Doppler frequency shift, and the relationship formula (formula (7)) of the rotating Doppler frequency shift and the azimuth angle of the rotating shaft is derived according to the scattering point model and the law of conservation of energy, and the azimuth of the rotating shaft of the object is determined. Under the condition that the rotating shaft of the object coincides with the propagation axis of the light beam, the theoretical central frequency shift values should be 2000Hz, 4000Hz and 6000Hz; under the misalignment condition, the three central frequency shift values change differently, and the azimuth of the rotating shaft can be determined according to the change rule of the rotating Doppler frequency shift value and the azimuth of the rotating shaft of the object. When the rotating shaft of the object is located in the first quadrant, When the rotating shaft of the object is located in the second quadrant, When the rotating shaft of the object is located in the third quadrant, When the rotating shaft of the object is located in the fourth quadrant, When the rotating shaft of the object is located in the third quadrant, When the rotating shaft of the object is located in the fourth quadrant, According to the judgment criterion, the specific quadrant position of the rotating shaft of the object can be determined.

[0040] In the experiment, the spliced vortex light beam contains three superposition state incomplete vortex light beams with different topological charge numbers, and the coverage range is In practice, more number of superposition state incomplete vortex light beams with different topological charge numbers with smaller coverage range can be used to construct different spliced vortex light beams, such as Figure 4 、 6 , to realize more fine division of the position of the rotating shaft of the object. At the same time, in long-distance transmission, the different topological charge number light beams in the spliced light beam will interfere with each other at the junction, so during the preparation of the spliced vortex light beam, a certain azimuth angle gap can be maintained between the different topological charge number light beams to reduce the interference, such as Figure 5 When the rotating object has a large lateral offset, the topological charge number interval of the spliced vortex light beam can be increased to make the frequency spectrum more clear.

[0041] The contents not described in detail in the present application belong to the prior art known to those skilled in the art.

Claims

1. A method for measuring the rotation axis based on a spliced ​​vortex beam: First, the phase structure of the beam is changed within different azimuth angle ranges to modulate a novel hologram of the spliced ​​vortex beam. Second, the emitted light from the laser is spatially filtered and polarized, and then irradiated onto a spatial light modulator loaded with the novel hologram to generate a spliced ​​vortex beam. Then, the spliced ​​vortex beam is irradiated onto a rotating object, and the scattered light is received by a photodetector and converted into an electrical signal. This signal is then sampled by a data acquisition card. Finally, based on the established relationship between the rotation axis position and the rotational Doppler frequency shift spectrum, and combined with the filtered and Fourier transformed rotational Doppler spectrum, the orientation of the rotation axis of the rotating object is determined.

2. The method for measuring the rotation axis based on spliced ​​vortex beams according to claim 1, characterized in that: In terms of vortex beam preparation, the phase structure of the beam can be adjusted within different azimuth ranges to obtain novel spliced ​​vortex beam holograms with multiple different topological charges within different azimuth ranges. The novel spliced ​​vortex beam is then prepared by irradiating a spatial light modulator loaded with the novel hologram using a polarized Gaussian beam.

3. The method for measuring the rotation axis based on spliced ​​vortex beams according to claim 1, characterized in that: Based on the non-coaxial rotating Doppler effect, an expression is established between the rotating Doppler spectrum value of the spliced ​​vortex beam as the light source and the azimuth angle of the object's rotation axis. By combining the rotating Doppler spectrum value obtained from sampling and processing the rotating object detected by the spliced ​​vortex beam, the azimuth angle of the object's rotation axis can be determined, and the position of the object's rotation axis can be identified through a single detection process.

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